384 



THE POPULAR EDUCATOR. 



accordingly divided into two classes, called respectively Great 

 Circles and Small Circles. Great circles are those whose plane 

 passes through the centre of the globe, so that they divide it 

 into two equal portions, and (assuming the earth to be perfectly 

 spherical) all these will be exactly equal. All other circles are 

 called small circles. 



The most important of the great circles is the equator, which 

 is an imaginary line drawn round the earth equally distant from 

 the north and south poles, and therefore dividing the globe into 

 two equal portions, called the northern and the southern hemi- 

 spheres. If now we imagine the plane of this circle to be ex- 

 tended to the sky, we shall have a great circle of the heavens 

 known as the celestial equator, or more usually the equinoctial. 

 This latter name is derived from two Latin words signifying 

 " equal " and " night," and is given to it because when the sun 

 appears to be on this line, it shines equally on each hemisphere, 



and day and night are then of equal length in all 



parts of the world, the sun being above the horizon 

 at every place for twelve hours, and below it for 

 the same period. The days on which this happens 

 are the 20th of March and the 23rd of September, 

 and by counting the days between these dates, we 

 shall find that in the northern hemisphere the sum- 

 mer is a few days longer than the winter, or, in 

 other words, that the period during which the sun 

 is north of the equator is longer than that during 

 which he is south of it. 



The sun's path is not, however, along the equi- 

 noctial, but in a great circle in- 

 clined to it at the present time 

 at an angle of about 23 27' 30", 

 and known as the ecliptic. Round 

 this the sun appears to travel, 

 performing the complete circuit 

 of it in the space of one year. 

 The space extending for nine de- 

 grees on each side of the eclip- 

 tic, and thus constituting a band 

 or belt 18 wide, is known as 

 the zodiac; and within this space 

 all the planets, with the excep- 

 tion of a few of the minor ones, 

 are constantly found, so that we 

 can always tell somewhat of the 

 position in which they are. As 

 already explained, the zodiac is 

 divided into twelve equal portions, each contain- 

 ing 30, and the stars in these spaces are mapped 

 out into the constellations known as the " signs of 

 the zodiac." 



We have just stated that the inclination of the 

 ecliptic to the equator, or, as it is called, the " obli- 

 quity of the ecliptic," was nearly 23^. This amount, 

 however, is not constantly the same, but varies a 

 little in the lapse of centuries. The rate of this variation is 

 very slight, being less than 1' in 100 years, and it is found 

 that it can only take place within very narrow limits. At pre 

 sent it is decreasing ; but before it can have deviated as 

 much as a degree and a half, the causes producing it will have 

 been so modified as to act in a contrary direction and increase the 

 inclination again. 



We shall see all through our lessons in Astronomy instances 

 of these slow and gradual variations ; but we shall find that all 

 are confined within certain very narrow limits, and that instead 

 of hurrying on the total change or destruction of the world and 

 the system to which it belongs, they tend to increase its 

 stability. 



Now since these two great circles are thus inclined, there 

 must be two points in which they intersect one another, and 

 these are called the equinoctial points, or the vernal and 

 autumnal equinoxes. One of these is the first degree in Aries, 

 and the other the first in Libra. The first of these, or the 

 vernal equinox, is the most important, as it is taken as the fixed 

 point to be employed in reckoning distances from when we 

 want to indicate the place of any body. 



We take, then, the equinoctial or equator as our base line, and 

 first of all measure the distance of any star north or south of 

 that. On a terrestrial globe, circles called parallels of latitude are 



usually drawn at distances of ten degrees. We can, however, 

 always measure the distance, by bringing the star to that side 

 of the brass meridian which is numbered upwards from the pole 

 to the equator, and reading off the degree. It must be remem- 

 bered when we speak of degrees of latitude that what we really 

 mean is the inclination which a straight line, drawn from the 

 place to the centre of the earth, would have to the plane of the 

 equator. A degree is a measure of an angle, and not of a dis- 

 tance. It is well to be clear on this point, as mistakes often 

 arise through want of understanding it. Some people will say 

 that a degree equals a little over sixty-nine miles, when in 

 reality what they mean is that at the equator two lines meeting at 

 the earth's centre, inclinedto one another as this angle, would in- 

 clude between them a portion of the earth's surface of that length. 

 On Saturn, or any globe larger than the earth, the amount 

 thus subtended at the equator would naturally be much greater; 



and on the other hand, in any small circles which 



G we may draw on a sheet of paper, there are still 

 360 degrees : each, therefore, is very minute. 



In astronomy, the distance north or south of 

 the equinoctial is called the declination of the 

 star ; and we have thus one of the two mea- 

 sures which we require to enable us to indicate 

 the star's place. If now we draw another great 

 circle passing through the poles, and also through 

 the star, it will intersect the equator in two 

 places, and the one of these on the same side as 

 that on which the star is situated will furnish 

 us with the other distance 

 required. If we examine the 

 equinoctial on a celestial globe, 

 we shall find that it is divided 

 into degrees, from to 360, 

 reckoning from east to west, the 

 starting-point being the first 

 point in Aries. 



The great circles which we 

 have referred to as passing 

 through the poles, are called 

 meridians, and any number of 

 them might be drawn ; usually, 

 however, twenty-four are drawn 

 on the globe,their distance apart 

 being fifteen degrees. They are 

 then frequently termed hour- 

 lines, as the sky appears to move 

 just the interval between two of them in the space 

 of an hour. We can obtain a clearer idea of these 

 meridians by taking the globe out of its frame- 

 work, and letting the brass meridian be free to 

 turn round on the poles; we can then bring it 

 over any star or place, and it will represent the 

 meridian of that place. We shall also be able 

 to see on the equator the distance of its intersection 

 from the first point in Aries. This distance is known as the 

 right ascension (usually abbreviated thus, R.A.). 



We see now the way in which we can determine the position 

 of a star, the two measures required being its right ascension 

 and its declination. Suppose, for example, we want to point 

 out the place of the star in the tip of the Great Bear's tail, we 

 first find it on the globe, and bringing it to the brass meridian, 

 we shall find that its elevation above the equinoctial is very 

 nearly 50 ; this then is its declination. We now look to the 

 equinoctial, and find the point of it directly under the meridian 

 is 204, or 13 hours 36 minutes from Aries ; and thus we assign 

 its place as 50 north declination, and 204 E.A. 



In a similar way, when the right ascension and declination 

 are given we can find the star. Thus, suppose we have the fol- 

 lowing question: What star has 99 33' K.A., and 16 29' 

 south declination ? We first find on the equinoctial the point 

 99 33', and bring this to the brass meridian. Keeping the 

 globe in this position, we now look under the degree 16 29 1 

 south, and there we find the bright star Sirius, or the Dog-star. 

 The student should practise finding in this way the position 

 of various stars, so as to render himself familiar with the use or 

 the globes. On a map, too, the meridians and parallels are 

 usually marked, so that these questions may be solved without 

 a globe, though it is more convenient to have one. 



G B 



